1. Field of the Invention
The present invention relates to a method for producing a resin molded article having openings, which exhibits an excellent surface appearance and satisfactory strength and relates to a resin molded article.
2. Description of Related Art
A resin molded article, having a plurality of openings, produced by molding synthetic resin by using a mold, for example; a wheel cap 9 for an automobile, as shown in FIG. 11, is known.
The wheel cap 9 comprises a central portion 91 formed at a position in the inside portion of the wheel cap 9, which is more adjacent to a resin-injection opening 2 than the position of the openings 95, an outer peripheral portion 92 formed outside of the openings 95 and bridge portions 93 formed between the adjacent openings 95.
The conventional mold for molding the wheel cap 9 comprises a plurality of projections for forming the openings 95, a plurality of bridge-forming portions for forming the bridge portions 93 formed between the projections, a central-portion forming portion for forming the central portion 91, and an outer-periphery forming portion for forming the outer peripheral portion 92.
The synthetic resin, in which glass fiber has been blended, is injected into the cavity of the mold through the injection opening so that the wheel cap 9 is produced.
However, the foregoing conventional producing method sometimes causes a weld 99 beside the openings 95 at a position in the outer peripheral portion 92 of the wheel cap 9 (see FIG. 11).
Formation of a weld 99 will specifically be described in relation to a wheel cap having draft holes for an automobile, for an example.
As shown in FIG. 11, the wheel cap 9 sometimes has a weld 99 having a width of 1 to 5 mm in the outer peripheral portion 92 located outside of the openings 95.
This weld 99 is formed because of a number of reason.
As shown in FIG. 12, a mold 8 for molding the wheel cap 9 has a cavity 82 including projections 81 for forming the openings 95. The synthetic resin 7 is injected through the resin-injection opening 2 positioned in the central portion 91 of the wheel cap 9. Therefore, the synthetic resin 7 moves in the cavity 82 for molding the wheel cap toward the outer peripheral portion 89, as shown in FIG. 12.
Since the cavity 82 includes the projections 81, the synthetic resin flow 7 branches to the right and left thereat (see FIG. 12) and then is joined in the rear of the projections 81 so that a conjunction portion 77 is formed (see FIGS. 12 to 14).
When the synthetic resin 7 moves forward in the cavity 82, the leading end of the synthetic resin flow forms a fountain flow, as shown in FIG. 13.
Accordingly, glass fiber 72 in the synthetic resin 7 is, in the vicinity of the conjunction portion 77, oriented in the direction of the conjunction portion 77, namely, in a direction substantially perpendicular to the surface of the wheel cap 9, as shown in FIG. 14.
At this time, the resin flow in the central portion of the cavity 82 also proceeds in a direction perpendicular to the direction of the thickness of the cavity 82. Accordingly the leading end of the resin flow is formed into a shape having the central portion which slightly projects, as shown in FIG. 15(B).
Since the thickness of the cavity 82 is substantially uniform, the amount of this projection is small. Thus, if the leading edges of the resin flow collide with each other at the projection portions, there is practically no space in which the flows can flow into the side portions. Therefore, the synthetic resin 7 substantially stands still on the collision.
The glass fiber 72 in the synthetic resin 7 on the collision is oriented in the direction substantially perpendicular to the surface 97 of the wheel cap 9, that is, in the direction of the thickness of the cavity 82.
Therefore, the synthetic resin flows 7 are joined to each other in this state. Accordingly, the glass fiber 72 is, in the conjunction portion 77 of the wheel cap 9 which is a produced article, oriented in a direction substantially perpendicular to the surface 97 of the wheel cap 9, as shown in FIGS. 14 and 15(A).
In a major portion of the wheel cap 9, the glass fiber 72 is oriented in the direction of the flow of the glass fiber 72, namely, along the surface 97 of the wheel cap 9.
Then, the synthetic resin 7 injected into the mold 8 is cooled, thus allowing the synthetic resin 7 to be coagulated and contracted. The contracting direction of the synthetic resin 7 is mainly the direction perpendicular to the direction in which the glass fiber 72 is oriented. Therefore, substantially no contraction generates in the conjunction portion 77 in which the glass fiber 72 is oriented perpendicularly to the surface 97. The surrounding portion in which the glass fiber 72 is oriented substantially in parallel with the surface 97 is contracted in the direction of the thickness.
As a result, the projecting weld 99 is formed on the surface 97 in the conjunction portion 77, as shown in FIG. 14.
Besides, since a surface solidified layer is inserted in the conjunction portion 77, there arises another problem in that the strength of the foregoing portion is lower than that of the other portion.
Such problems are not limited to the wheel cap. Various resin molded articles, such as a bumper and garnish for an automobile, each having openings sometimes encounter the similar problems.
In view of the foregoing, an object of the present invention is to provide a method for producing a resin molded article having openings, which is free from having a weld in a conjunction portion of synthetic resin flows and which exhibits an excellent surface appearance and satisfactory strength, and for providing a resin molded article obtained thereby.
According to a first aspect of the present invention, a method for producing a resin molded article having openings includes the step of injection-molding synthetic resin by using a mold. The mold for molding the resin molded article includes a cavity which has projections for forming openings, projection-side peripheral portions surrounding the projections, and a flow-rate changing means for making the flow rate of the injected synthetic resin different at both sides of each projection and wherein the synthetic resin is injected into the cavity of the mold through a resin-injection opening so that molding is performed.
The key feature of the present invention is that the cavity of the projection-side peripheral portion surrounding projections comprises the flow-rate changing means for making the flow rate-of the injected synthetic resin different at the both sides of each projection.
Examples of the flow-rate changing means are a means for making the cross-sectional areas of the cavity different, a means for making the temperature of the cavity different, a means for providing obstacles, such as a dam with a part of the cavity, and a means for providing a groove formed along the synthetic resin flow at the bottom of the cavity.
The operation and effect of the present invention will now be described.
When the resin-molded article is produced, the synthetic resin is injected into the cavity of the mold through the resin-injection opening.
The synthetic resin is sequentially fed to the entire cavity.
The synthetic resin collides with the projections of the cavity and moves forward along projection-side peripheries of the both sides of the projections. Then, the resulting branch flows of the synthetic resin are again joined at the rear of the projections.
The xe2x80x9crear of the projectionsxe2x80x9d means a position opposite the resin-injection opening.
The cavity of the projection-side peripheral portions surrounding the projections comprises the flow-rate changing means. Thus, the flow rates of the synthetic resin flows, which move forward around the projections along both sides thereof, are different from each other.
Therefore, the flow rates of the synthetic resin flows, which are joined, are different from each other. Further, the leading ends of the synthetic resin flows move forward, forming a fountain flow.
Subsequently, when a head-on collision of the synthetic resin flows occurs, the central portion of the synthetic resin, having a high pressure and a high speed, is inserted into the central portion of the synthetic resin, which flows at a lower speed.
Accordingly, in the portion adjacent to the surface of the resin-molded article, the conjunction portion is formed into an acute-angled shape almost parallel with the surface of the resin-molded article (see FIG. 4(C)).
When, for example, an inorganic filler is mixed with the synthetic resin, the inorganic filler adjacent to the surface of the resin-molded article is oriented substantially in parallel with the surface also in the conjunction portion. This is because that the conjunction portion is formed substantially in parallel with the surface of the resin-molded article in the portion adjacent to the surface of the resin-molded article whereas the inorganic filler adjacent to the conjunction portion is oriented along the conjunction portion.
Therefore, generation of the weld of the resin-molded article can be prevented.
The conjunction portion is formed into an acute-angled shape almost parallel with the surface of the resin-molded article in the portion adjacent to the surface thereof. Thus, only a small stripe groove is formed on the surface.
Moreover, the conjunction portion of the synthetic resin is not formed into a planar shape, such as in the conventional resin-molded article, but is formed into a shape in which the central portion of either of the synthetic resin is inserted into the central portion of the other synthetic resin.
Hence, the strength of the conjunction portion of the resin-molded article can be improved.
As described above, the present invention can provide a method for producing a resin-molded article having openings and exhibiting an excellent surface appearance and satisfactory strength without generation of a weld at the conjunction portion of the synthetic resin flows.
According to a second aspect of the present invention, a method for producing a resin-molded article having openings, includes injection-molding synthetic resin by using a mold to produce a resin-molded article comprising a central portion formed in an inner portion of the cavity at a position more adjacent to the resin-injection opening than the openings, an outer peripheral portion formed at a position outside of the openings, and bridge portions formed between the adjacent openings, wherein the mold for molding the resin-molded article comprises a plurality of projections for forming the openings, a plurality of bridge-forming portions for forming the bridge portions, a central-portion forming portion for forming the central portion, and an outer-periphery forming portion for forming the outer-peripheral portion wherein the adjacent bridge-forming portions have different cross-sections, and wherein the synthetic resin is injected into the cavity of the mold through the resin-injection opening so that molding is performed.
An essential feature of this invention is that the adjacent bridge-forming portions of the mold have different cross-sections.
In producing the resin-molded article, the synthetic resin is injected into the cavity of the mold through the resin-injection opening.
Thus, the synthetic resin moves forward inside the cavity in a direction towards the outer-periphery forming portion from the central-portion forming portion through the bridge-forming portion.
At this time, the resistance between the synthetic resin and the surface of the mold in the bridge-forming portion having the small cross-section is higher than that in the bridge forming portion having the large cross-section. For this reason, the synthetic resin moves at a higher speed in the bridge-forming portion having the large cross-section as compared with the synthetic resin which moves in the bridge-forming portion having the small cross-section.
The synthetic resin flows introduced into the outer-periphery forming portion area from the bridge-forming portion are again joined together in the outer periphery forming portion.
At this time, the speeds of the synthetic resin flows which are joined to each other are different. The leading edge portions of the resin flows move forward as fountain flows.
Thus, when a head-on collision of the synthetic resin flows occurs, the central portion of the synthetic resin having a high pressure and high""speed is inserted into the central portion of the synthetic resin which flows at a lower speed.
Therefore, the conjunction portion is formed into an acute-angled shape almost parallel with the surface of the resin-molded article in the portion adjacent to the surface thereof (see FIG. 4(C)).
When, for example, an inorganic filler is mixed with the synthetic resin, the inorganic filler in the portion adjacent to the surface of the resin-molded article is oriented substantially in parallel with the surface also in the conjunction portion. This is because the conjunction portion is formed substantially in parallel with the surface of the resin-molded article in the portion adjacent to the surface thereof whereas inorganic filler adjacent to the conjunction portion is oriented along the conjunction portion.
Therefore, generation of the weld of the resin-molded article can be prevented.
The conjunction portion is formed into the acute angle almost parallel with the surface of the resin-molded article in the portion adjacent to the surface thereof. Therefore, only a small stripe groove is formed on the surface.
Moreover, the conjunction portion of the synthetic resin is not formed into a planar shape such as in the conventional resin-molded article, but is formed into a shape in which the central portion of either of the synthetic resin is inserted into the central portion of the other synthetic resin.
Accordingly, the strength of the conjunction portion of the resin-molded article can be improved.
The difference in the cross-sectional area between the adjacent bridge-forming portions is adjusted to adjust the difference in the speed of synthetic resin which flows in the bridge-forming portions. This enables the position of the conjunction portion of the synthetic resin flows discharged as mentioned above to be adjusted.
Thus, the conjunction portion can be formed at a position, for example, at a corner of the opening, which is an inconspicuous part of the resin-molded article (see FIG. 5).
As described above, according to the present invention, generation of a weld can be prevented in the conjunction portion of the synthetic resin flows. Thus, it is possible to provide a method for producing a resin-molded article having openings and exhibiting an excellent surface appearance and satisfactory strength.
According to a third aspect of the present invention, a method for producing a resin-molded article having openings includes injection-molding synthetic resin by using a mold, wherein the mold for molding the resin-molded article comprises a cavity which has projections for forming the openings, projection-side peripheral portion surrounding the projections and projection-separated portions distant from the projections, wherein the cavity has flow-rate changing means for making the flow rate of the injected synthetic resin different at the projection-side peripheral portion and at the projection-separated portion, and wherein the synthetic resin is injected into the cavity of the mold through a resin-injection opening so that molding is performed.
The operation and effects of the producing method will now be described.
The synthetic resin injected through the resin-injection opening collides with the projections and then branched into both sides of each projection.
The cavity has the flow-rate changing means. Hence, the synthetic resin flows move forward at different speeds at the projection-side peripheral portion and the projection-separated portion of the mold.
Thus, when the synthetic resin branched into both sides are again joined in the rear of the projection, the synthetic resin is sequentially joined at the projection-side peripheral portion or at the projection-separated portion of the mold which respectively corresponds to the peripheral portion and adjacent portion of the resin-molded article (see FIG. 10).
Therefore, the synthetic resin has a space in which the synthetic resin can flow toward the projection-separated portion or the projection-side peripheral portion even after the synthetic resin has been joined.
In a case where the inorganic filler is mixed with the synthetic resin, the inorganic filler is temporarily oriented in the direction of the thickness of the cavity when the synthetic resin has been joined in a head-on collision manner, forming a fountain flow.
However, since the synthetic resin is able to flow out to the projection-separated portion even after the synthetic resin has been joined to each other, the synthetic resin is temporarily oriented in the direction of the thickness again flows. The inorganic filler is oriented in a direction in which the synthetic resin is flows, namely, in a direction parallel with the surface of the resin-molded article.
This prevents generation of a weld in the conjunction portion.
As described above, according to this method, it is possible to provide a method for producing a resin-molded article having openings, which exhibits an excellent surface appearance without generation of a weld in the conjunction portion of the synthetic resin flows.